Postion of Menu/Hamburger on Home Page

I understand how the menu on the home page will change to a hamburger at a certain page width, but I am having trouble understanding the trigger that moves the menu from a location to the right of the header & image to a location centered beneath them. It is this latter location that is producing the off-screen bleed in the left-hand sub-menus so many are complaining about. It occurs in both Firefox and IE but not Chrome. I can force the hamburger to appear in Firefox by narrowing the screen width; and I can produce a menu location to the right of the homepage header by widening the screen width (or actually by Changing the Size of All items in Display to 100%); but my default display of 125% with 1280×1024 res [yes, I have a small monitor] is putting the menu in the problematic position.

The website in question is still in process but can be viewed at: https://crfg.org/WordPress/

Is there a way to fix this?